The embodiments described herein relate generally to systems that effect a transfer function on an input signal to produce an output signal that ideally has a predetermined relationship to the input signal, such as amplifier systems, and, more particularly, to a system linearization assembly that may be used with such signal handling systems.
Signal processing systems, such as amplifier systems, are configured to modify or change at least one parameter of a signal. For example, an amplifier system may be configured to receive an input signal and to increase the amplitude of the signal, preferably without inducing other changes to the signal in the process. The amplified signal or the output signal can then be transmitted to a transmission line or to an antenna, for example.
However, at least some amplifier circuits and similar systems exhibit at least some form of non-linear behavior, which may result in a distortion of the amplified or output signal. Such a distortion may decrease the overall performance of the amplifier system. For example, a harmonic distortion in an electronic amplifier introduces abnormalities in the output signal at the fundamental input frequency (w) and at multiples of the input frequency (2w, 3w, etc.) that constitute the inherent distortion. Some harmonic distortion can be filtered out by using bandpass filters. Intermodulation distortions (IMD), however, include the difference of the frequencies in the output, and can be difficult to filter out of the amplified or output signal. More specifically, in the case of two closely spaced input signals (w1 and w2), the resulting intermodulation (IMD) component frequency for a 3rd order intermodulation (IM3) and other odd terms is immediately adjacent to the signals. When the resulting IMD component frequency is immediately adjacent to the signals in the frequency spectrum, it can be difficult to filter out. The intensity of an nth order intermodulation distortion component is proportional to the intensity of the nth order harmonic distortion component, provided the impedance of the output circuit is the same at both frequencies.
At least some methods have been proposed for linearization of amplified or output signals. For example, one method is known as the “power backoff” method. The idea is to reduce the input signal amplitude so as to avoid driving the amplifier output into distortion. Such techniques control distortion and noise at the expense of amplification efficiency. Other known methods include pre-distortion and post-distortion methods to cancel the effect of the amplifier's non-linearity.
These conventional methods seek to minimize the adverse effects of the non-linearity but fail to correct the nonlinearity of the amplifier itself. Instead, such methods merely “cover” or “patch” the non-linearity such that the adverse effects of the non-linearity can be less apparent.
The nonlinearities in transistor amplifiers are strongly dependent on the quiescent voltage and current bias settings at which the amplifier operates. For instance, by changing the bias voltages supplied to a transistor amplifier, the 3rd order nonlinear distortion can be reduced or increased. This allows for nonlinear distortion to be minimized without significantly compromising the gain in the system.